1. Field of the Invention
The present invention relates generally to a cardiac pacemaker sense amplifier that discriminates between actual heart signals and extraneous repetitive noise signals.
2. Description of the Prior Art
Demand cardiac pacemakers are well known to those familiar with the art. Such pacemakers provide a heart stimulating pulse to the patient's heart only in the absence of an actual heartbeat and are inhibited should the natural heartbeat recur at a rate exceeding a preset demand pacing rate. Such pacemakers are commonly of the ventricular inhibited type which are designed to sense a QRS complex within the escape interval programmed for its basic rate and to recycle without producing a ventricular stimulating pulse but may take the form of an atrial inhibited pulse generator, atrial synchronous, ventricular inhibited pulse generator, atrial or ventricular synchronous triggered pulse generator, or atrial-ventricular sequential pulse generator. Such pacemakers are described in the article entitled "The Report of Inter-Society Commision for Heart Diseases Resources, Implantable Cardiac Pacemakers Status Report and Resource Guideline" published by the magazine Circulation, Vol. L, October 1974. A further, more recent pacemaker, employs both atrial and ventricular sense amplifiers and pulse generators and operates in a fully automatic mode, as described in co-pending U.S. patent application Ser. No. 235,069, filed Feb. 17, 1981, in the name of L. Herpers and assigned to assignee of the present invention. Such an automatic pacemaker possesses timing circuitry for providing atrial and ventricular escape intervals and the A-V timing interval and coupling circuitry which causes the timing circuitry to be reset upon sensing either an atrial heart depolarization, or P wave, or a ventricular heart depolarization, or R wave. The amplitude and frequency components of the P wave and the R wave differ from one another and the atrial and ventricular sense amplifiers must accordingly be designed to respond to the particular heart signal. To complicate matters, the P and R waves may be detected in either chamber and retrograde R-waves may mimic P-waves in the atrium.
In practice, it has been found that other signals generally referred to as electro-magnetic interference or EMI may possess frequency components, amplitudes and slew rates which can likewise be interpreted as a natural heart signal. For example, strong electrical signals commonly encountered in the environment, such as stray 60 Hz energy, may result in the inhibiting of the stimulating pulse as if a natural heartbeat had not been detected. To prevent the inhibiting of a stimulating pulse by extraneous noise, typical prior art demand cardiac pacemakers have been provided with noise rejection circuitry, shielding, or reversion circuitry to convert the operation of the pacemaker into a fixed rate or asynchronous mode of operation.
In the U.S. Pat. No. 3,927,677 assigned to the assignee of the present invention, a reversion circuit is described which is capable of detecting actual heart activity in the presence of extraneous repetitive noise. That circuit has proven to substantially alleviate the effects of EMI and to allow the pacemaker to discriminate between actual heart signals and the background repetitive noise.
However, the application of the reversion circuitry of U.S. Pat. No. 3,927,677 to atrial and ventricular pacemakers causes special problems arising from the fact that the atrial or ventricular sense amplifiers must be blanked or disconnected for blanking intervals commencing on the production of a respective ventricular or atrial pacing stimulus to prevent that stimulus from being detected as a natural heart depolarization and from overloading the sense amplifier input stages. The disconnection and subsequent reconnection of the input stages of the sense amplifier presents a special problem to the reversion circuitry found useful in the discrimination between electromagnetic interference and natural heart signals. The initial noise impulse upon reconnection could be mistakenly interpreted as a natural heart depolarization.